can't give you linux advice - would be interested in hearing from a completely linux school.
However - £8000 is not cost of a TA - they cost more than that when you add in national insurance contributions from school etc.
Half your cost probably is office - you would only need MS office on your terminal servers to run sims.
How much is a stable system worth to your school - if IT goes down most schools cease to function now so be very careful in changing.

